1. Field of the Invention.
The present invention pertains generally to semiconductor fabrication and more specifically to multilevel integrated circuit metallization processes and devices.
2. Description of the Background.
The formation of conductors and other structures in a semiconductor using multilevel metallization processes typically employ aluminum that is deposited using conventional techniques such as sputtering. Although aluminum is simple and easy to work with in these processes, it has certain disadvantages when employed in a semiconductor device.
Aluminum is particularly susceptible to electromigration problems As current passes through aluminum structures formed in a semiconductor, the atoms of the aluminum tend to migrate because of the low bonding strengths between the atoms. This is also reflected in the low melting temperature of aluminum. In very small structures in the semiconductor, e.g., one micron wide conductors, the migration of the aluminum atoms in response to the flow of current can cause an open circuit. Due to the effect of electromigration, strict limits must be imposed as to the minimum size of structures formed from aluminum in a semiconductor circuit.
Aluminum also tends to corrode quite easily in a semiconductor circuit. Typically, the pattern of structures is formed through a process of etching using a chlorine based etcher. If any of the chlorine is left behind after the etching process is completed, it acts as a catalyst to corrode the aluminum layer. For example, if any aluminum chloride remains it will react with water to form aluminum oxide, aluminum hydroxide and hydrochloric acid, which in turn combines with the aluminum in the circuit to form aluminum chloride, such that the process continues in a progressive fashion to corrode the aluminum in the circuit. Because of the complex structures that exist in most semiconductor circuits, it is extremely difficult to remove all of the aluminum chloride to prevent deterioration of the circuit due to corrosion of the aluminum.
The methods of depositing aluminum using physical vapor deposition techniques such as sputtering and evaporation can also cause problems in the resulting semiconductor device. For example, shadowing and geometric effects occur when aluminum is deposited using conventional physical vapor deposition techniques. In other words, deposition of aluminum by the process of sputtering over a structured topology having, e.g., steps and deep holes on the surface of the semiconductor, will many times not provide a uniform covering of the uneven surface topology. More specifically, physical vapor deposition techniques will not provide a uniform deposition of aluminum atoms on the back side of tall structures and aluminum atoms will not be deposited in holes such that a uniform surface coverage is provided. When the surface is etched, discontinuities are likely to occur because of the nonuniform coverage.
Another problem that occurs because of the nonplanarity of the substrate is that stringers, i.e., unetched portions of thick layers of the aluminum surrounding steps on the semiconductor surface, are not eliminated during the etching process so that a short circuit is produced between adjacent aluminum structures. In the process of etching the aluminum, the etching process is allowed to continue to fully etch the flat portions of the aluminum which are inherently much thinner than the thicker portions that occur around steps on the semiconductor. If the stringer exists between two adjacent conductors, for example, a short circuit will result. To eliminate stringers, overetching is employed to ensure that all the aluminum is removed in the etching areas. However, the process of overetching, in this manner, to remove stringers, will many times result in undercutting and other degradation of the aluminum structure, as well as damage to other surfaces, such as semiconductor surfaces on the semiconductor chip The undercutting process, i.e., the removal of aluminum at the interface of the aluminum and the surface in which it is deposited, can also cause discontinuities and reduce the width of the aluminum structure to further intensify electromigration problems and increase line resistance.
Additionally, to ensure conductivity for vias, i.e., connections between two vertically disposed metal conductors in a semiconductor circuit, and between contacts, i.e., connection between a vertically disposed metal layer and a silicon layer, dogbones are typically employed. A dogbone comprises a widened portion of the circuit to ensure full horizontal registration between the layers that are designed to be conductively connected. When working in the one micron region, full horizontal registration between vertically disposed layers does not always occur. In many cases, a trench or deep hole will form at the contact or via adjacent to the underlayer to which the connection is to be made. Since sputtered aluminum will not penetrate and deposit in deep holes or trenches, proper contact is not made unless there is enclosure of the via. Hence, the widened portions, or dogbones, are provided to ensure that full horizontal registration occurs and adequate contact is made. The disadvantage of using dogbones is that they limit the density of the circuit since the electromigration problems of aluminum inherently limit the minimum size of the aluminum structures in the circuit. In other words, the minimum width of the aluminum structures is increased since widened portions must be provided to ensure horizontal registration and full conductive contact at vias and contacts.